The present invention relates to a cathode ray tube mainly used for receiving TV broadcasts and a glass bulb for a cathode ray tube.
As is shown in FIG. 2, a cathode ray tube 1 primarily used for receiving TV broadcasts has an envelope basically formed by bonding a panel portion 3 as an image display and an almost funnel-shaped funnel portion 2 which comprises a neck portion 5 housing an electron gun 11, a yoke portion 6 for mounting a deflection coil and a body portion 4, along a sealing portion 10. The panel portion 3 consists of a skirt portion 8 to be joined with the funnel portion 2 and a face portion 7 as an image display. The panel portion 3 and the funnel portion 2 make up a glass bulb.
In FIG. 2, 12 denotes a phosphor layer which emits fluorescence upon irradiation with an electron beam, 14 denotes a shadow mask which defines the positions of the phosphors to be irradiated with an electron beam, and 13 denotes a stud pin to fix the shadow mask 14 to the inside of the skirt 8. A is the tube axis which leads the central axis of the neck portion 5 to the center of the panel portion 3. The face portion 7 of the panel portion 3 is a substantially rectangular area surrounded by four edges substantially parallel with the long and short axes which intersect at right angles on the tube axis A.
A cathode ray tube maintains a high vacuum in it to display images made of luminescence from phosphors excited by high speed electron bombardment. The difference between the internal and external pressures of the glass bulb acts as an external force to produce a vacuum stress on the aspherical and asymmetric glass bulb, and a great tensile stress, or a tensile vacuum stress, develops on the edges of the face portion of the panel portion, the outer surface of the skirt portion and the outer surface of the funnel portion near the sealing portion. The tensile vacuum stress is especially great at the ends of the short and long axes of the panel portion on the edges of the face portion (the ends of the axes of the face portions).
FIG. 3 shows a stress distribution along the short and long axes, and the solid line represents the vacuum stress in the paper plane, while the broken line represents the vacuum stress perpendicular to the paper plane. The numbers affixed to the stress distribution lines represent the magnitudes of the stress at the respective spots. FIG. 3 clearly shows that the tensile vacuum stress is generally great along the short axis, the panel portion has a maximum stress on the edges of the face portion, while the funnel portion has a great stress near the sealed edge of the body portion. A thinner glass bulb suffers a larger tensile vacuum stress and is more likely to mechanically fracture upon abrasion of these regions where the stress reaches a maximum.
A crack in a glass bulb for a cathode ray tube in such a state spreads to release the high internal deformation energy to fracture of the bulb. Besides, a glass bulb with a high tensile stress on the outer surface may be less reliable because delayed destruction can take place due to the action of the atmospheric moisture. Though a simple way to secure mechanical strength of a glass bulb is to increase the thickness of the glass bulb sufficiently, this ends up with an increase in weight to about 37 kg in the case of a glass bulb with a screen size of about 76 cm.
On the other hand, numerous image displaying devices other than the cathode ray tube have come into practical use in recent years. As compared with them, the great depth and weight of the cathode ray tube is pointed out as its big disadvantage as a displaying device. Therefore, there is strong pressure to reduce the depth or weight. However, reduction in the depth of a conventional cathode ray makes its structure more asymmetrical and therefore causes the problem of accumulation of more deformation energy in the glass bulb. Further, weight reduction usually leads to increase in deformation energy by making the glass less rigid, and the resulting higher deformation energy helps increase the risk of fracture and reduce reliability against delayed destruction by producing a large tensile stress. Increase of the glass thickness prevents the stress from increasing by lowering the deformation energy, but results in increase of weight, as described above.
As a conventional way to reduce a glass bulb for a cathode ray tube in weight, it is practical to form a compressive stress layer on the surface of the glass panel in ⅙ the thickness of the glass by physical tempering, as disclosed in U.S. Pat. No. 2,904,067. However, it is impossible to uniformly quench the panel portion and the funnel portion having three-dimensional structures and uneven thicknesses. Since a large residual tensile stress develops concurrently with the compressive stress due to the uneven temperature distribution, the compressive stress is limited to at most about 30 MPa, and it is impossible to produce a relatively large compressive stress. In summary, reduction of the weight of a glass bulb by physical tempering is limited because the resulting compressive stress is relatively small.
It is also known to reduce the weight of a glass bulb by chemically tempering its surface. In this method, specific alkali ions in the glass are replaced with larger ions at temperatures below the annealing temperature, and the resulting volume increase causes formation of a compressive stress layer on the surface. For example, strontium-barium-alkali-alumina-silicate glass containing from 5 to 8% of Na2O and from 5 to 9% of K2O is immersed in molten KNO3 at about 450xc2x0 C. Chemical tempering is advantageous over physical tempering in that it can provide a large compressive stress about from 90 MPa to 300 MPa without producing an undesirable tensile stress.
On the other hand, as compared with physical tempering, chemical tempering is disadvantageous in that because it usually provides a relatively thin compressive stress layer of about from 20 xcexcm to 200 xcexcm, which is about the same as the depth of abrasions made during manufacture of cathode ray tubes or on the market, a compressive stress layer having an insufficient thickness has little effect against abrasions having depths greater than its thickness. Formation of a sufficiently thick compressive stress layer requires that the glass be maintained at nearly annealing temperature for a long time and therefore has problems of deformation of the glass and of stress reduction due to stress relaxation. Further, it has been unclear how much the weight of a glass bulb can be reduced by chemical tempering in view of the magnitude of the stress and the thickness of the resulting compressive stress layer, while securing sufficient reliability, i.e., the limitation of weight reduction.
The object of the present invention is to solve the drawbacks of the conventional techniques for weight reduction of glass bulbs. Namely, in the above-mentioned conventional weight reduction of glass bulbs by chemical tempering, the thickness of the compressive stress layer formed by chemical tempering is determined simply from the depth of abrasions anticipated during manufacture of cathode ray tubes or on the market, and the influence of the tensile vacuum stress which develops on the glass bulb due to the difference between the internal and external pressures of the cathode ray tube on the compressive stress layer is not considered at all. Namely, the relationship between the tensile vacuum stress and the effective thickness of a compressive stress layer has not been sufficiently elucidated yet.
Therefore, no glass bulb with light weight which sufficiently resists abrasions anticipated during manufacture of cathode rays or on the market even under a tensile vacuum stress is available, and its realization is strongly demanded.
In view of the above-mentioned problems and object, the present invention provides a glass bulb which is enough reliable to sustain the difference between the internal and external pressures of a cathode ray tube, by determining the weight reduction of a glass bulb by chemical tempering from the relationship between the maximum tensile vacuum stress resulting from the difference between the internal and external pressures of a cathode ray tube which depends on the structure and the wall thickness of the glass bulb and the thickness of the compressive stress layer resulting from the chemical tempering and the magnitude of the compressive stress in the region where the maximum tensile vacuum stress occurs.
The present invention provides a glass bulb for a cathode ray tube comprising a panel portion having a substantially rectangular face portion and a funnel portion having a neck portion, wherein when the glass bulb is used for a cathode ray tube, the glass bulb at least regionally suffers from a tensile stress resulting from the atmospheric pressure on the outer surface of the glass bulb having a vacuum inside, at least part of the face portion of the panel portion where the tensile stress over the face portion has a maximum value "sgr"VP has a compressive stress layer formed by chemical tempering on the outer surface, and the "sgr"VF, the magnitude of the compressive stress on the compressive stress layer "sgr"C MPa, and the thickness of the compressive stress layer tC xcexcm satisfy the following relationship:
120/tCxe2x89xa7(1xe2x88x92|"sgr"VP/"sgr"C|) greater than 30/tC 
provided that "sgr"vpxe2x89xa720 MPa.
The present invention also provides a glass bulb for a cathode ray tube comprising a panel portion having a substantially rectangular face portion and a funnel portion having a neck portion, wherein when the glass bulb is used for a cathode ray tube, the glass bulb at least regionally suffers from a tensile stress resulting from the atmospheric pressure on the outer surface of the glass bulb having a vacuum inside, at least part of the funnel portion where the tensile stress over the funnel portion has a maximum value "sgr"VF has a compressive stress layer formed by chemical tempering on the outer surface, and the an, the magnitude of the compressive stress on the compressive stress layer "sgr"C MPa, and the thickness of the compressive stress layer tC xcexcm satisfy the following is relationship;
120/tCxe2x89xa7(1xe2x88x92|"sgr"VF/"sgr"C|) greater than 30/tC 
provided that "sgr"VFxe2x89xa710 MPa.
The present invention further provides a cathode ray tube using the glass bulb for a cathode ray tube.